The present invention broadly relates to a magnet suitable for use as the rotor magnet of a stepping motor and, more particularly, to a cylindrical permanent magnet having a surface multipolar anisotropy.
In order to effect a multipolar magnetization on the surface of a permanent magnet, it is essential that the permanent magnet has a high coercive force and that the reversible magnetic permeability is around 1. To meet these demands, hitherto, ferrite magnets such as barium ferrite magnet, strontium ferrite magnet and so forth have been used as the permanent magnet for multipolar magnetization. Materials of such permanent magnets are stoichiometrically expressed by the MO.multidot.6Fe.sub.2 O.sub.3, where M represents an element such as Ba, Sr, Pb or their mixtures with or without bivalent metal such as Ca. Actually, however, the MO content is somewhat excessive so that the materials of such permanent magnets are expressed by MO.multidot.nFe.sub.2 O.sub.3, where n represents a value of 5 to 6.
Various cylindrical permanent magnets have been proposed and used hitherto, such as isotropic ferrite magnet, ring anisotropic ferrite magnet, and plastic magnets formed by dispersing ferrite magnetic powder in a matrix such as synthetic rubber, synthetic resin or natural rubber.
These known permanent magnets, however, suffer from various disadvantages as follows. Namely, the isotropic ferric magnet, which is usually produced by compacting, cannot provide satisfactory magnetic properties, since magnetic rubber is oriented in direction at random. The ring anisotropic magnet 11 as shown in FIG. 1 has a radial particle orientation as shown by arrows B. The magnetization after the sintering, however, is conducted by means of a magnetizing yoke A composed of a yoke member 1 formed of ferromagnetic material and coils 2 as shown in FIG. 2. Therefore, the direction of particle orientation and the direction of line of magnetic force indicated by arrows C locally discord with each other. Thus, the ring anisotropic magnet 11 cannot provide effective particle orientation. Such ring anisotropic ferrite magnet is described in U.S. Pat. Nos. 3,114,715 and 4,057,606. In the plastic permanent magnet 12 as shown in FIG. 3, the particle orientation coincides with the directions of lines of magnetic force indicated by arrow C. This plastic magnet 12, however, cannot provide sufficient amount of magnetic flux because of small residual magnetic flux density. Such plastic magnet is described in Japanese Laid-Open Patent Publication No. 57-130407.
In addition, the conventional cylindrical permanent magnet could not avert from the problem of large inertia force which is caused inevitably by increased wall thickness for obtaining a high magnetic properties.